This invention relates to an overcurrent protection device, and more particularly to an overcurrent protection device for protecting electrical appliances from overcurrent.
In motor driven electrical machines and devices when moving parts are clogged and stop due to the accumulation of dust and ice, or the action of outside forces, overloading occurs which causes a current flow much higher than the rated value with the consequence that coils or other parts may burn. As is known in the art and shown in FIG. 13, an overcurrent protection device P is placed in the current sourcing circuit of the electrical machine, such as a motor M, and this overcurrent protection device P can cut off the circuit when the current becomes higher than a prescribed level.
In the prior art, a current-type fuse was used as this type of overcurrent protection device P. As shown in FIG. 14 for motor M, which has a rated operating current of I.sub.R : if the motor "locks up" at time t.sub.f and becomes overloaded, overcurrent I.sub.l flows and the resulting heat opens current-type fuse P, and the electrical current in the circuit is cut off. In this way, as current-type fuse P cuts off the electric circuit, electric motor M is protected. A major drawback, however, with the use of a current-type fuse is the fact that each time the electric circuit is cut off, a new current-type fuse must be installed, a rather tedious process for the user.
A thermostat and temperature-type fuse may be used as a means to replace the current-type fuse. Such devices can detect overheating of an electrical machine and then cut off the electric circuit. However, with conventional thermostats, after the electric circuit is cut off, the electric circuit often is closed (reconnected) again whether or not the aforementioned electric machine has sufficiently cooled. Consequently, there can be a cycling overcurrent flow situation which is a problem. In addition, faulty operation may take place when the surrounding temperature rises, although no current actually flows. For the temperature-type fuse, just as in the case of the current-type fuse, each time the electric circuit is cut off, a new fuse must be installed. This, of course, is an inconvenience.